(Original post by I Have No Imagination)
Please...I'm still on frikkin meiosis!
I used a revision guide for most of it and sometimes the textbook. The textbook takes so long hence learning rev guide then checking for things i missed. then i failed jan so ive gotta learn all that again -.-
Used to produce gametes which are haploid, when they fuse together it becomes a zygote which is diploid and so they divide and develop. Happens in the reproductive organs - ovaries and testes.
2 stages - Meiosis 1 and 2, in one there is only one cell where its visible, for the 2nd stage theres 2 cells visible and it happens at right angles to meiosis 1.
Interphase happens which replicates DNA and organelles then:
Homologous pairs come together to form a bivalent. One chromosome is maternal and the other is paternal. Genes are at the same loci on these.
Prophase 1:Non-sister chromatids (e.g. ones with diff alleles) cross over at points called chiasmata (these can brak off and be swapped around so causing genetic variation - called resassortment or recombination due to crossover or crossing over).
The nuclear envelope breaks down and the nucleous disappears too. Centrioles at poles of a cell begin to form spindle fibres.
Metaphase 1: The bivalents line up randomly along the spindle fibre equator which leads to random segregation during anaphase (called random assortment of chromosomes). The spindle fibres attach to the centromere of EACH chromosome in the homologous pair
Anaphase 1: The spindle fibres contract which pulls one member chromosome from each homologous pair goes to the poles of the cell. Centromeres do NOT divide. Where there are chiasmata, the crossed over parts go with the chromosome the new one they joined too.
Telo 1: In animal cells, the genetic material has a nuclear envelope formed around it and the cells then divide by cytokinesis and chromosomes then uncoil.
In plant cells they go straight to metaphase 2.
Pro 2: If a nuclear envelope present, it breaks down and spindle fibres begin to form and chromosomes condense.
Meta 2: Random assortment of the chromatids along spindle fibres equator allows random segregation during ana 2 so greater genetic variation. Spindle fibres attach at centromeres.
Ana 2: spindle fibres shorten and chromosomes pulled apart at centromeres, each chromatid goes to the opposite poles of the cells (depending on assortment which side).
Telo 2: Animals -> nuclear envelope forms and then they divide to form 4 cells.
Plants --> a tetrad of cells is formed straight away.
In both all cells are now haploid and have much greater variation